NOVORY

Paint Calculator

Estimate how much paint you need for any wall, including deductions for doors and windows.

Quick answer

2 gallons for two coats (1 gallon covers a single coat with a small margin).

Calculator

ft
ft

How it works

Wall length × Wall height = Total wall area

The calculator multiplies the wall's length by its height to get the total surface area. This is the starting point before accounting for openings.

Subtract doors and windows

Each standard door deducts 21 sq ft and each standard window deducts 15 sq ft from the total wall area, giving you the paintable surface.

Paintable area × Coats ÷ 350 = Gallons

The paintable area is multiplied by the number of coats, then divided by 350 — the typical coverage of one US gallon. The result is also converted to liters (1 gallon ≈ 3.785 L).

Worked example

A 20 ft × 9 ft wall with 1 door, 2 windows, and 2 coats:

20 × 9 = 180 sq ft

180 − 21 − 30 = 129 sq ft

129 × 2 = 258 sq ft

258 ÷ 350 = 0.74 gal

≈ 2.40 liters

Buy approximately 1 gallon (or 2.5 L) of paint.

Email me my estimate

Get this Paint Calculator estimate plus our free Paint Color Planner in your inbox.

Free Paint Color Planner PDF included. No spam, unsubscribe anytime.

Paint is the cheapest material in the project and the most annoying to run out of. A half-painted wall can't wait for a second trip — the new gallon will flash differently and you'll see the seam forever.

This calculator works the way pros estimate: total wall area, minus a standard 21 sq ft per door and 15 sq ft per window, times the number of coats, divided by real-world coverage of about 350 sq ft per gallon. Two coats is the default because one coat over primer — let alone over a color change — is a gamble.

How to measure walls for paint

  1. Measure each wall's length and height in feet and add them up — or enter one representative wall if they're identical. Measure to the ceiling, not to where you think you'll cut in.
  2. Count every door and window. The calculator deducts 21 sq ft per door and 15 sq ft per window, the standard estimator's allowances.
  3. Set the coats honestly: 2 for a color change or new drywall, 1 only for a same-color refresh over sound paint.
  4. Read the result in gallons (and liters) — then buy the full amount in one trip. Paint from two different batches can vary in sheen.

Coverage per gallon by surface (one coat)

SurfaceCoverage per gallon
Smooth drywall (repaint)350–400 sq ft
New or porous drywall250–300 sq ft
Textured walls / stucco200–250 sq ft
Brick / concrete block150–200 sq ft

Manufacturer labels quote the smooth-surface number. Rough or thirsty surfaces can drink 30–50% more — prime first or budget accordingly.

Worked example: paint for a 12×12 room

Four 12-ft walls, 8-ft ceilings, one door, two windows, two coats:

  1. Wall area: 4 × 12 × 8 = 384 sq ft.
  2. Deductions: 1 door × 21 + 2 windows × 15 = 51 sq ft.
  3. Paintable per coat: 384 − 51 = 333 sq ft.
  4. Two coats: 333 × 2 = 666 sq ft.
  5. Gallons: 666 ÷ 350 = 1.90 → buy 2 gallons.

2 gallons for two coats (1 gallon covers a single coat with a small margin).

Pro tips for buying paint

  • Porous surfaces — new drywall, bare plaster, brick — need primer or ~25% more paint. Skipping primer to save money is the most expensive saving in painting.
  • Box your paint: if you're using more than one gallon per color, mix them together in a 5-gallon bucket so the color is uniform wall to wall.
  • Ceilings get ceiling paint (flat, spatter-resistant), not leftover wall paint — the sheen difference shows in raking light.
  • Save the label (or a photo of it) with the leftover can. Touch-ups with the exact formula and sheen are invisible; close-enough never is.

How to use the paint calculator

1. Choose your unit system — Imperial (feet) or Metric (meters).

2. Enter the length of the wall you plan to paint.

3. Enter the height of the wall.

4. Enter the number of coats — two is typical for most projects.

5. Enter the number of doors and windows on the wall so their area is deducted.

6. Click Calculate to see how much paint you need in gallons and liters.

How paint is calculated

The calculator first determines the total wall area by multiplying the wall length by the wall height. It then subtracts the area of doors and windows — a standard door is 21 sq ft and a standard window is 15 sq ft — to get the paintable area.

The paintable area is multiplied by the number of coats. The result is divided by the coverage per gallon (350 sq ft per gallon) to get the amount in gallons, and by 10 sq m per liter to get liters.

Paint coverage explained

Paint coverage varies by brand and type, but a general rule is that one US gallon covers about 350 square feet with one coat. Premium paints may cover up to 400 sq ft, while thinner or darker paints may only cover 250–300 sq ft.

Always check the label on your paint can for the manufacturer's stated coverage. If the coverage differs from 350 sq ft per gallon, adjust your order accordingly.

Paint calculator example

Suppose you are painting a wall that is 20 feet long and 9 feet high, with 1 door and 2 windows, and you want 2 coats.

Total wall area: 20 × 9 = 180 sq ft.

Deduct doors and windows: 180 − (1 × 21) − (2 × 15) = 180 − 21 − 30 = 129 sq ft.

Apply 2 coats: 129 × 2 = 258 sq ft of coverage needed.

Gallons: 258 ÷ 350 = 0.74 gallons.

Liters: 258 sq ft ≈ 23.97 m². 23.97 ÷ 10 = 2.40 liters.

Buy approximately 1 gallon or 2.5 liters of paint for this wall.

Frequently Asked Questions

How much paint do I need per square foot?+
One US gallon of paint typically covers about 350 square feet with a single coat. That means roughly 1 gallon per 350 sq ft per coat. If you are applying two coats, you will need twice the paint. Always check the manufacturer coverage on your specific paint can, as it can vary.
How do I calculate the paintable wall area?+
Multiply the wall length by the wall height to get the total wall area, then subtract the area of any doors and windows. A standard interior door is about 21 square feet and a standard window is about 15 square feet. The calculator does this deduction automatically.
How many coats of paint do I need?+
Two coats are recommended for most projects, especially when changing colors or painting over a darker shade. One coat may be sufficient when repainting with the same color in good condition. The calculator multiplies the paintable area by the number of coats you enter.
How do I convert gallons to liters?+
One US gallon equals approximately 3.785 liters. The calculator provides paint requirements in both gallons and liters so you can use whichever unit your paint is sold in.
Should I buy extra paint?+
It is a good idea to buy about 10% more than the calculated amount to account for touch-ups, roller absorption, and touch-up after drying. Keeping leftover paint also allows you to match the color later if needed.

Related Guides

How to Calculate How Much Paint You Need for a RoomHow Much Paint Do I Need for a 12x12 Room?How Many Gallons of Paint Do I Need per Square Foot (Exterior)?How Much Paint Do I Need for a 10x12 Bedroom?

Free Home Project Toolkit

Printable worksheets and checklists for measurements, materials, and budgets.

Get Free Toolkit
Embed this calculator

Copy this code to embed the calculator on your own website or blog:

<iframe src="https://novory.studiotools.workers.dev/embed/tools/home/paint-calculator/" width="100%" height="640" style="border:0" title="Paint Calculator by NOVORY"></iframe>